The transformed properties of five hybrid cell lines which had either one or another parent in common were studied and compared with their tumorigenicity. Three hybrid cell lines, derived from the Chinese hamster DC-3F/ADX/Aza line, were resistant to actinomycin-D. This property seemed to be correlated with the presence of a marker chromosome from the common parent. The tumorigenicity was intermediate between those of the parent cell lines. On the other hand, agglutinability by concanavalin A (Con A) was variable. Three hybrid cell lines which had either the A9 or the clone 1D (both derived from mouse fibroblasts) showed very similar transformed characteristics, but two were tumorigenic and one not so. It appears from this study that the properties of the hybrid cell lines can be influenced more by one parent, depending on the genes retained at chromosome segregation. The limits of Con A agglutination as a characteristic of transformation and the validity of the check pouch grafts as tumorigenicity test for malignant human cell lines are discussed.